1. Field of the Invention
The invention relates to methods of manufacturing rotary drill bits, and particularly rotary drag-type drill bits of the kind comprising a bit body having a threaded shank for connection to a drill string and a leading face on which are mounted a plurality of cutters secured to the bit body by brazing.
2. Description of Related Art
The cutters may, for example, be preform cutting elements comprising a layer of superhard material, such as polycrystalline diamond, bonded to a substrate of less hard material, such as cemented tungsten carbide. The substrate of the cutting element may be bonded, for example by brazing, to a carrier which may also be of cemented tungsten carbide, the carrier then being brazed within a socket on the leading face of the bit body. Alternatively, the substrate of the cutter may itself be of sufficient size to be brazed directly within a socket in the bit body.
Drag-type drill bits of this kind are commonly of two basic types. The bit body may be machined from metal, usually steel, and in this case the sockets to receive the cutters are formed in the bit body by conventional machining processes. Alternatively, the bit body may be formed using a powder metallurgy process in which a metal mandrel is located within a graphite mold, the internal shape of which corresponds to the desired external shape of the bit body. The space between the mandrel and the interior of the mold is packed with a particulate matrix-forming material, such as tungsten carbide particles, and this material is then infiltrated with a binder alloy, usually a copper alloy, in a furnace which is raised to a sufficiently high temperature to melt the body of the infiltration alloy and cause it to infiltrate downwardly through the matrix-forming particles under gravity. The mandrel and matrix material are then cooled to room temperature so that the infiltrate solidifies so as to form, with the particles, a solid infiltrated matrix surrounding and bonded to the metal mandrel.
Sockets to receive the cutters are formed in the matrix by mounting graphite formers in the mold before it is packed with the particulate material so as to define sockets in the material, the formers being removed from the sockets after formation of the matrix. Alternatively or additionally, the sockets may be machined in the matrix. The cutters are again secured in the sockets by brazing.
In order to braze the cutters in place the cutters are located in their respective sockets with a supply of brazing alloy. The bit body, with the cutters in place, is then heated in a furnace to a temperature at which the brazing alloy melts and spreads by capillary action between the inner surfaces of the sockets and the outer surfaces of the cutters, an appropriate flux being used to facilitate this action.
In use, such drill bits are subject to considerable wear and erosion, for example due to the high pressure flow of drilling fluid over the surface of the bit body for the purpose of cooling and cleaning the cutters and transporting the cuttings to the surface. Matrix-bodied bits are very resistant to such wear and erosion but it is desirable for a steel bodied bit to be hardened and/or formed with a harder erosion-resistant coating.
During the process of brazing the cutters to the bit body, the bit body must be heated to a temperature which is usually in the range of 500.degree.-750.degree. and with the steels hitherto used in the manufacture of the bit bodies of rotary drag-type bits, the heating/cooling cycle employed during brazing of the cutters in position has the effect of reducing the hardness and strength of the steel. In view of this, it is common practice to manufacture the steel body of the bit, or the steel mandrel of a matrix bit, in two parts. In the case of a steel bodied bit, one part of the bit provides the leading surface to which the cutters are brazed while the other part provides the threaded shank for connection to the drill string. The two parts are welded together after the cutters have been brazed to the leading part so that the shank part is not reduced in hardness or strength as a result of the brazing process.
Similarly, in a matrix bodied bit the steel mandrel is formed in two parts. A first part is mounted within the mold so that the solid infiltrated matrix may be bonded to it and the second part of the mandrel, providing the threaded shank, is welded to the first part after the matrix has been formed and after the cutters have been brazed into the sockets in the matrix. The part of the mandrel providing the shank does not therefore have its hardness or strength reduced by the brazing process nor by the heating/cooling cycle of the infiltration process.
It would be desirable to avoid the necessity of forming the bit body or mandrel in two parts. Forming the bit or mandrel in two parts not only adds to the cost of the manufacturing process but the necessity of welding the parts together may compromise the design of the bit body. For example, the bit body must be of sufficient length, and so shaped, as to provide a region where the two parts can be welded together. Accordingly, a one-piece bit body could be shorter in length than a two-piece body and this may have advantage, particularly where the drill bit is for use in steerable drilling systems. Furthermore, the reduction in hardness and strength of the first part of the bit body or mandrel during brazing, or infiltration and brazing, is in itself undesirable and in the case of a steel bodied bit makes it even more necessary to hard face the surface of the bit body to avoid unacceptable wear or erosion. Also, the reduction in strength can lead to premature failure of the bit, particularly due to fracture of the blades on which the cutters are mounted.
The present invention is based on the realisation that by using a different type of steel, or other metal alloy, from those hitherto used for rotary drag-type drill bit bodies, the heating/cooling cycles to which the bit body is subjected during manufacture may be used actually to harden and strengthen the alloy of the bit body rather than reduce its hardness and strength, with the result that the bit body, or mandrel in the case of a matrix-bodied bit, may be formed in one piece, thus avoiding the above-mentioned disadvantages.
This is achieved by using for the bit body or mandrel a precipitation hardening alloy, which may, for example, be steel or stainless steel. As is well known, a precipitation hardening alloy is an alloy in which very fine particles of constituents of the alloy may be caused to precipitate, i.e. initiate and grow from the parent alloy, so as to harden and strengthen the alloy. Such precipitation may be effected by subjecting the alloy to a controlled heating and cooling cycle.
The initiation and growth of precipitates "precipitation" is a diffusion process, i.e. it is controlled by time and temperature. A certain threshold amount of energy is required to trigger initiation. In certain alloys, there is sufficient energy at room temperature to trigger initiation; albeit at a very slow pace. In the majority of alloys, however, an elevated temperature, and a minimum time at that temperature, is required to trigger initiation.
The size of the precipitates is critical to the degree of hardness, strength, and ductility obtained. The precipitation hardening effect arises from the precipitates causing local distortion of the crystal lattice. The greatest hardness (and the lowest ductility) is achieved when the precipitates are numerous and exceptionally fine. As the temperature is increased above a threshold temperature, larger and fewer particles are precipitated and, as a result, hardness decreases and ductility increases. As the temperature is raised further, there comes a point where the particles are too few and too large to contribute appreciably to the hardness/strength of the alloy.
A "solution" heat treatment in which the alloy is raised to an even higher temperature, acts to "dissolve" the majority of existing precipitates, by taking them back into the solid solution. Subsequent cooling to room temperature tends to lock the precipitation hardening elements into solid solution. The faster the cooling rate, the greater is this tendency. The slower the cooling rate, the more chance there is to initiate and grow precipitates during the cooling cycle. The precipitates created during the cooling cycle, from the higher temperature, tend to be less beneficial to increasing hardness/strength than those created by a subsequent, separate, precipitation hardening heat treatment.
The overall aim, according to the invention, therefore, is to subject the alloy from which a bit body is formed to a combination of time and temperature which causes precipitation hardening and gives rise to the optimum hardness/ductility combination. In theory, this may be achieved by first taking all the precipitates into solution at a high "solution treatment" temperature; followed by fast cooling to room temperature; followed by heating quickly to a lower precipitation hardening temperature and holding at that temperature for a prescribed time; followed by a fast cool back to room temperature. Precipitation hardening may also be effected by performing the latter precipitation hardening step alone.
The present invention lies in controlling the heating/cooling cycle to which the bit body or mandrel is subjected during the process of brazing the cutters to the bit body, so as to effect precipitation hardening, or, in a matrix-bodied bit, controlling the heating/cooling cycle to which the bit body is subjected during the infiltration process, so as to effect a preliminary "solution" heat treatment, as described above, prior to the precipitation hardening effected by subsequently brazing the cutters to the bit body.